Shown above are the the pistons listed in the DATSUN/NISSAN Competion Parts Book.
On the Left - is the Forged Piston - Part No. 99996-M10xx, according to the book it yeilds around 12.5:1 to 13:1 on the L-28 engines with the E-31 Head.(yeiks - not for street use today!)
On the Right - is the Factory Cast Pistion - Part No. 12010-E31xx (L24) and 12010-A87xx (L26). Combined with the E31 head - this will yeild aprox. 11:1 to 11.5:1 compression ratio (still to high for street use with todays gas!).
This is the Cast Piston - NISSAN Part No. 12010-P86xx. It has a 2cc off-set dish, to keep the compression ratio below 10.5:1 (depending on the head used and the amount of milling done to the head or decking to the block).